Ceramic insulating composition



Dec. 8, 1942. w H, GERlsCH 2,304,562

CERAMIC INSULATING COMPOSITION Filed Dec. 18, 1941 Inventor: Walter H. Gevisch,

His Akbar-neg.

Patented Dec. 8, 1942 Walter H. Gerisch, North to General Electric Company, a corporation of New York Berton, N. 1., assignor Application December 18, 1941, Serial No. 423,413

4 Claims.

This is a continuation-m-part of my prior application S. N. 373,440, filed January '7, 1941, for Liquid contact circuit closers, said application being assigned to the General Electric Company, a corporation of New York.

The present invention relates to an improved ceramic insulating composition.

For many purposes in the electrical arts it is desirable to employ an insulator which is characterized not only by satisfactory electrical properties but also by relatively high mechanical strength and ability to resist heat shock; Such applications include, for example, circuit-interrupting barriers for use in mercury switches and electrode supporting structures for use in discharge tubes, such as fluorescent lamps. It is a primary object of the present invention to provide a refractory ceramic insulating material which is well adapted for these applications.

In general, this object is accomplished by the provision of a ceramic comprising a fused composition of magnesia (magnesium oxide) in major proportion, at least of a borosilicate glass, and a complementary proportion of clay.

The features of the invention which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the drawing, in which Fig. 1 is a sectional view of a mercury switch incorporating a ceramic part produced in accordance with'the invention; Fig. 2 is a side elevation of a discharge lamp illustrating another use of the invention; and Fig. 3

I is a fragmentary sectional view illustrating in detail certain of the parts of the lamp of Fig. 2.

The construction of Fig. 1 comprises a mercury switch formed of a pair of cup-shaped metal parts I, I, which are constituted, for example, of

. chrome-iron alloy. These parts are separated by a ceramic barrier 2 and are hermetically sealed to one another by a glass ring 3, the ring being also in sealing contact with the outer periphery of the barrier 2. The switch contains a charge of hydrogen or a mixture of hydrogen and air and encloses a quantity of mercury indicated at 5. In the closed position of the switch, the mercury forms a conducting path between the parts I, I through an aperture I formed centrally in the barrier 2. When the switch is tilted on the other hand, the mercury is divided into two separate pools by the barrier so that the switch circult is broken.

It is important for the successful functioning of the switch that the ceramic barrier 2 be of a composition which is capable of withstanding the destructive effects (especially heat shock) to which it is subjected because of the arcing condition which accompanies either opening or closing of the switch. In addition, it is necessary from a constructional standpoint that the ceramic material employed have substantially the same coefficient of expansion as that of the sealing ring 3.

Where the parts I, I are of chrome-iron alloy. it has proven desirable to form the sealing ring 3 of a relatively high expansion glass (e. g. a glass having an expansivity on the order of 9 10- centimeters per centimeter per degree Centigrade), this expansivity being provided, for example, by a glass having a composition within the following range:

Per cent PbO 20-30 Alkali Oxide 10-16 'SiOz Remainder One example of such a glass is that sold by the In accordance with the present invention, a ceramic which is of highly refractive character and which at the same time sufllciently matches a sealing glass such as that described above is made of magnesia (a material of very high expansivity) in combination with appropriate proportions of borosilicate glass and clay. In the preferred case, a ceramic of the desired quality is prepared by fusing a mixture of magnesia and a vitrifiable binder including both borosilicate glass and clay, the relative proportions of the combined materials being adjusted to produce a coefllcient of expansion of the ceramic as a whole which is matched to that of the sealing glass.

In this connection it is to be noted that while typical borosilicate glasses have expansion coeflicients on the order of 3X 10- centimeters per centimeter per degree centigrade, magnesia has an expansion coeflicient above 14x10-. Consequently, a substantial quantity of borosilicate glass may be combined with magnesia without producing a resultant expansivity too low for proper cooperation with the sealing glass.

a In a particular case, I have found that a highly refractory ceramic having a coefiicient of expansion closev to that of a lead sealing glass, such as Corning 001, may be produced by combining powdered fused magnesia with clay and with a glass of the .Pyrex type. For the latter constituent one may use, for example, a borosilicate glass having approximately the following oxide composition;

, Percent SiO2 80.5 B203 12.9 NaaO 3.8 K20-.. 0.4 A1203- 2.2

This is a typical analysis of the glass sold by the Coming Glass Company under their code No. 174.

The compositionof the ceramic may be varied within limits approximately as follows: Per cent Magnesia -65-75 Glass 5-20 Clay"; Remainder Per cent Magnesia u 75 Ball clay 15 No. 7'74 glass 10 ,In combining the materials the clay is first added to the magnesia, both of these being preferably powdered to 200 mesh fineness. Thorough admixture of the ingredients is secured by tumbling them for approximately two hours in a ball mill. To render this combination readily moldable one may add afew percent of a temporary binder such, for example, as paraflin wax heated to its melting point. The mixed batch is transferred to a dough mixer and heated to the melting temperature of the wax, whereupon the hot liquid wax is added to the batch and the mass is kneaded until the batch has cooled down to room temperature. Thereafter dry powdered borosilicate glass of 200 mesh fineness is added and the batch is further kneaded for or minutes to insure thorough dispersal of the glass powder through the batch. Portions of this composition may be placed in molds of desired shape and size and subjected to sufiicient pressure to assure a compactly molded product. Thereafter the molded articles are fired at a temperature of between 1100 and 1200 C." (preferably about 1150 C.) to remove the temporary binder and to vitrify and fuse the clay and glass ingredients, thus binding all the parts together. A firing time of as little as three minutes at 1150" C. has been found to be sufficient to produce a highly durable, hard, uniform and non-porous product.

Barriersconstituted as described in the foregoing are capable of being sealed in place in a circuit closer of the type under consideration without cracking the sealing glass by which the metal parts of the device are joined, this result being a consequence of the correlation of the expansivity of the ceramic with the corresponding property of the sealing glass. In addition, due to the presence of the borosilicate glass component, such barriers are highly resistant to heat shock and to the erosive action of arcs formed during circuit making and breaking operations.'

The improvement in this latter respect over otherwise similar barriers in which the glass constituent is omitted is so great as to double the current which may be interrupted by a given form of barrier without excessive erosion of the barrier surfaces.

A further application of my improved ceramic is illustrated in Fig. 2, which represents a selfrectifying lamp of the fluorescent type, the central portion of the lamp being broken away in order to conserve space in the drawing. The lamp, which may be filled with an appropriate combination of a vaporizable metal, such as mercury, and a fixed gas, such as argon, comprises an elongated glass tube III. This is coated internally with a fluorescent material of known type and has at one end a re-entrant stem press I l bearing a filamentary cathode 12. At the other end there are provided a pair of identical graphite anodes [5, I5 adapted to receive a discharge from the cathode l2 on alternate half cycle The anodes are separated by means'of a metalli shield I1 and are supported from a common ste press l8 through which lead-in wires l9 ar sealed. In order to prevent the discharge from striking to the lead-in wires and from damaging the stem press, the rear extremity of each of the anodes is enclosed in a ceramic part 2!, 2| of cup-shaped configuration.

The details of the anode mounting are illustrated in Fig. 3, which shows one of the cupshaped parts 2 l 'as being secured to the inner end of a lead-in wire I9 by means of a metal retainer 23 disposed within the insulator and attached to the extremity of the lead-in wire. The part 23 includes a threaded extension 23' which engages a correspondingly threaded bore in the anode l5. A glass sleeve 25 surrounding the lead-in wire l9 and fused to it serves to facilitate sealing the the stem press and the operation of the lamp.

It is found, however, that all of these various conditions are successfully resisted when the parts are molded from the composition of my invention, that is to say, a composition of magnesia in fused relation with borosilicate glassand clay in the range of proportions hereinbefore indicated.

In addition to the applications specifically described in the foregoing, other uses of my improved ceramic will occur to those skilled in the art. I aim in the appended claims to cover all variations of composition or use which are within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure b Letter Patent of the United States is:

l. A refractory silica insulator comprising vitrified mixture of magnesium oxide in major proportion no less than about 65 per cent, a borosilicate glass present to the extent of at least about 5 per cent, and a complementary proportion of clay.

2. A ceramic consisting of a fused composition of from 65 to magnesia, at least 5% of a low expansion borosilicate glass, and a complementary proportion of clay.

3. A ceramic consisting essentially of a composition of from 65.to 75% magnesia, from 5 to of a low expansion borosilicate glass.

WALTER H. GERISCH. 

